PHANEROGAMIA : AXIAL ORGANS. 241 



distinct, as in Ribes, Alnus, Elceagnus, Juglans, Populus, Salix, Carpinus, 

 Castanea, Corylus, Quercus, Cytisus, Cornus mascula, Sambucus, Rham- 

 nus, Tilia. 5. Finally, I have found this layer either entirely absent, 

 or only to be recognised in the external cellular layer, as in Cheiranthus, 

 Hippophde, Mesembryanthemum, and the so-called tree-carnations. On the 

 whole, the external cortical layer seems to stand in a definite relation to 

 the formation of cork, and to be more sharply defined in proportion to the 

 tardiness with which the latter appears (as, for instance, in Cactacece, 

 Aristolochia Sipho, Cacalia ficoides) : the contrary, however, also occurs, 

 as, for instance, in Mesembryanthemum. 



2. Perennial Bark. The development of the vascular bundles from 

 the cambium is always accompanied by a similar development of the 

 bark, since a part of the cells that have newly originated in the cambium 

 attach themselves, towards the interior, to the vascular bundle, a second 

 part continues to develop as cambium, and the remainder attaches itself 

 externally to the old bark. Thus we find that, similarly to the annual 

 rings of the wood, definite layers of bark are formed in every period of 

 vegetation, being composed, according to the peculiar character of the 

 primary bark, of mere parenchyma, liber and parenchyma, or of alternate 

 layers of parenchyma and liber, or of alternating layers of true paren- 

 chyma and such as is interrupted by liber-bundles. From this the layer 

 of liber frequently becomes broader towards its inner side, in proportion 

 as the wood thickens, so that the liber-bundles exhibit a beautiful wedge- 

 like appearance in a cross-section. This new formation of bark is, 

 however, marked by great specific differences : in some plants it is rapid 

 and decided, as, for instance, in the Lime-tree ; in others very slow and 

 partial, as in the Beech. The thickness of the bark depends partly 

 upon this, and partly upon the following causes : sometimes, even in the 

 first period of vegetation, and then generally uniformly, the epidermis 

 is developed into a cork-tissue (as in most trees) ; less frequently this 

 occurs later, in that case beginning at separate spots, and extending 

 itself by degrees, as, for instance, in the Cacti and leafless Euphorbiacece 

 ( 29.). The cork-tissue varies in hardness and durability. It 

 most frequently consists of tabular cells, already described in the 

 First Book, which are sometimes thickened in alternate layers, as, for 

 instance, in the Cacti ; and more rarely somewhat radially elongated, as 

 in the Cork Oak, or Cork Elm. In the last named, and in the Maple, it 

 acquires considerable thickness, but at the same time is easily destroyed 

 by atmospheric influences in the case of the Maple. In its usual form it 

 generally lasts longer, and becomes frequently very thick, constituting 

 the so-called bark of the tree, as, for instance, in Quercus Robur. 

 Occasionally the cork is found in the bark, in the condition of separate 

 layers of an easily destroyed tissue, in which case it falls off in horizontal 

 bands, or shreds, having a specifically definite form. 



In some stems new layers of a parenchyma, very similar to cork-tissue, 

 are developed from the cambium ( ?) with cortical parenchyma (in Ribes\ 

 or alternately with cortical parenchyma and liber (as in the Vine) : this 

 parenchyma (termed periderma by H. Mohl) likewise contains easily 

 destroyed layers, so that the whole external bark drops off, and then, 

 as in the case of cork-formation, layers of periderma and liber are 

 successively shed. (This is strikingly exemplified in Pinus sylvestris.} 

 We are, however, very deficient in the necessary investigations. We 

 have to thank H. Mohl for the first exact work on this point.* I have 



* On the Origin of Cork and Bark. Tubingen, 1835. 

 R 



